1. Field of the Invention
The present invention relates to a piezoelectric element for use in, for example, an actuator for changing capacity of ink chamber in order to jet ink filled in the ink chamber from an ink nozzle through an ink passage in an ink-jet printer head, a process for producing the piezoelectric element, and an ink-jet printer head which is constructed using the piezoelectric element.
2. Prior Art
An ink-jet printer head is generally composed of a head base, a diaphragm, and an actuator. A part of the head is enlargedly and schematically shown in FIG. 5. A head base 11 is provided with a large number of ink nozzles for jetting ink, a large number of ink passages separately communicating to the respective ink nozzle, and a large number of ink chambers 12 separately communicating to the respectively ink passages. (FIG. 5 shows only one of the ink chambers 12, and illustration of the ink passages and the ink nozzles is omitted.) A diaphragm 13 is mounted so as to cover the whole upper face of the head base 11, and the diaphragm 13 closes upper face openings of all the ink chambers 12 of the head base 11. On the diaphragm 13, piezoelectric elements 15 for giving vibration driving force to the diaphragm 13 are mounted and formed on positions separately corresponding to the respective ink chambers 12. A power source 19 of an actuator 14 provided with a large number of piezoelectric elements 15 is controlled and voltage is applied to a desired and selected piezoelectric element 15, whereby the piezoelectric element 15 is displaced and the part of the diaphragm 13 is vibrated. As a result, capacity of an ink chamber 12 which is located at the part corresponding to the vibration of the diaphragm 13 is changed, and ink is pushed out from the jet nozzle through the ink passage.
Each of the piezoelectric elements 15 is constructed by forming a piezoelectric film 17 on a lower electrode 16 and forming an upper electrode 18 on the piezoelectric film 17 so as to sandwich the piezoelectric film 17 between the lower electrode 16 and the upper electrode 18. The piezoelectric film 17 is generally made of lead titanate zirconate (Pb(Zr, Ti)O3; PZT) or made of a material mainly composed of PZT. The piezoelectric film 17 having such composition is formed by sputtering method, vacuum deposition method, CVD method, laser ablation method, sol-gel method, thick film method (a method using piezoelectric paste), and so on.
For example, the Japanese Patent Publication (unexamined) No. 217458/1998 discloses a process for forming a piezoelectric film made of PZT, in which an organic raw material of PZT is spin coated on a lower electrode, dried, and degreased, and after repeating the process of applying the organic raw material, drying, and degreasing several times, burning is conducted and a piezoelectric film is obtained. When a PZT film to be a piezoelectric film is prepared by the mentioned method, a low dielectric substance, which has pyrochlore structure or is amorphous, whose dielectric constant is lower than that of the PZT film consisting of crystals of perovskite structure is formed in a grain boundary exposure region of the piezoelectric film. This Japanese Patent Publication (unexamined) No. 217458/1998 describes that an excess composition produced in the process of growing crystals by burning the piezoelectric film is transferred to a grain boundary along with the growth of PZT crystal grains, and when the neighboring crystal grains finally join to close the grain boundary, the excess composition is pushed out on a surface layer face through the grain boundary, and the low dielectric substance is thus formed. This Japanese Patent Publication (unexamined) No. 217458/1998 explains that the low dielectric substance is formed in the grain boundary exposure region of the piezoelectric film, and consequently, the voltage applied on the crystal grain boundary of the piezoelectric film at the time of applying and electric field is lowered as compared with the voltage in a case where the low dielectric substance is not formed and leakage current flowing through the grain boundary of the piezoelectric film is decreased, and therefore voltage-proof characteristics of the piezoelectric element are improved.
However, in the method in which the leakage current of the piezoelectric element is restrained by forming the low dielectric substance, which has pyrochlore structure or is amorphous, in the grain boundary exposure region of the piezoelectric film as disclosed in Japanese Patent Publication (unexamined) No. 217458/1998, it is difficult to control the process so that the low dielectric substance may be constantly formed at all times in a uniform state in the grain boundary exposure region of the piezoelectric film or on substantially the whole face of the piezoelectric film, and therefore it is difficult to obtain a piezoelectric element which has uniform piezoelectric characteristics at all times.
The present invention was made to resolve the above-discussed problems and has an object of providing a piezoelectric element in which the piezoelectric element has uniform and excellent piezoelectric characteristics at all times and process control in the production of the piezoelectric element is relatively easy. Another object of the invention is to provide a process for suitably producing such a piezoelectric element. A further object of the invention is to provide an ink-jet printer head in which the mentioned piezoelectric element is used as an actuator.
An invention according to claim 1 provides a piezoelectric element in which electrodes are arranged on both sides of a piezoelectric film respectively, in which the piezoelectric film is provided with an oxide layer containing 0 or not more than 15 weight percent of Pb arranged on a face of the piezoelectric film, the face being in contact with at least one of the electrodes, and said oxide layer being formed of a composite oxide expressed by a chemical formula ABO3 or of a solid solution of one or not less than two kinds of composite oxides respectively expressed by the chemical formula ABO3.
An invention according to claim 2 provides the piezoelectric element according to claim 1, wherein the oxide layer is formed of: a composite oxide a expressed by the chemical formula, ABO3, in which A is one or not more than two kinds of elements selected among a group of alkaline-earth metals and Pb and B is one or not more than two kinds of elements selected among a group of Ti, Zr, and Sn; a composite oxide b expressed by the chemical formula, ABO3, in which A is one or at least two kinds of elements of alkaline-earth metals and B is Nb and/or Ta; or a solid solution of the composite oxide a and the composite oxide b.
An invention according to claim 3 provides the piezoelectric element according to claim 1 or 2, in which an oxide forming the oxide layer is a ferroelectric material.
An invention according to claim 4 provides the piezoelectric element according to any of claims 1 to 3, in which the oxide layer contains not more than 5 weight percent of Pb.
An invention according to claim 5 provides the piezoelectric element according to any of claims 1 to 4, in which the piezoelectric film is formed of PZT expressed by a chemical formula, Pb (Zr1-xTix)O3 (0.1xe2x89xa6xxe2x89xa61) or formed of a material mainly composed of PZT.
An invention according to claim 7 provides the piezoelectric element according to claim 1, in which the oxide layer is 0.05 xcexcm to 1 xcexcm in thickness.
An invention according to claim 8 provides the piezoelectric element according to claim 1, in which the piezoelectric film is 1 xcexcm to 25 xcexcin thickness.
An invention according to claim 9 provides the piezoelectric element according to claim 1, in which the oxide layer is not more than 10% of the piezoelectric film in thickness.
In the piezoelectric element according to claim 1 or 2, the piezoelectric film is provided with an oxide layer which does not contain lead or an oxide layer which contains a small amount of lead, and the oxide layer is arranged on a face of the piezoelectric film, the face being in contact with the electrode. Consequently, the leakage current is decreased as compared with a piezoelectric element which is not provided with an oxide layer. It is possible to form the oxide layer by forming a film on the piezoelectric film or on the electrode, and therefore process control in the production of the piezoelectric element is relatively easy.
In the piezoelectric element according to claim 1, mechanical durability of the element is more improved, and the element is effectively prevented from breakdown or the like due to peeling of the piezoelectric element and the electrode. This is probably because, as a result of employing the oxide layer being formed of a composite oxide expressed by a chemical formula ABO3 or of a solid solution of one or not less than two kinds of composite oxides respectively expressed by the chemical formula ABO3, a superior lattice matching is obtained between the oxide layer and the piezoelectric film and electrodes. In the piezoelectric element according to claim 2, a furthermore improved mechanical durability of the element is achieved. This is probably because the lattice matching is improved all the more between the oxide layer and the piezoelectric film and electrodes. In addition, in each piezoelectric element according to claims 1 and 2, mechanical durability of the element is more improved.
In the piezoelectric element according to claim 3, the piezoelectric film is provided with an oxide layer which is ferroelectric, and the oxide layer is arranged on a face of the piezoelectric film, the face being in contact with the electrode. As a result, it is possible to lower driving voltage of the piezoelectric element.
In the piezoelectric element according to claim 4, leakage current is decreased more and, as a result, superior piezoelectric characteristics are performed.
In the piezoelectric element according to claim 7, it is possible to securely obtain the advantage of decreasing the leakage current because the oxide layer is not less than 0.05 xcexcm in thickness, and performance as a piezoelectric element is not deteriorated because the oxide layer is not more than 1 xcexcm in thickness.
In the piezoelectric element according to claim 8, the piezoelectric film is 1 xcexcm to 25 xcexcm in thickness. As a result, there is no problem that effective vibration is not achieved by the element due to excessively thin piezoelectric film and deficient driving force and that a large driving voltage is required for displacement of the element due to excessively thick piezoelectric film.
In the piezoelectric element according to claim 9, the oxide layer is not more than 10% of the piezoelectric film in thickness, and therefore the piezoelectric characteristics to serve as a piezoelectric element are not deteriorated. When the oxide layer is excessively thick, ratio of the piezoelectric film to the whole piezoelectric element is smaller, and this cause deterioration in characteristics of the piezoelectric element such as increase in driving voltage otherwise decrease in displacement amplitude of the element. However, such deterioration does not occur when the oxide layer is not more 10% of the piezoelectric film in thickness.